The present invention relates generally to high temperature superconductors. More specifically, the present invention relates to high grade thallium based superconductors.
Of course, the phenomenon of superconductivity has been known for a number of years. In contrast to normal electrical conductors, when an electrical field is applied to a superconducting material electrons flow with no measurable resistance.
Normal electrical conductors, such as copper or silver, permit electrons to flow easily within the bulk metal when an electrical field is applied. However, in bulk copper, for example, most of the atomic electrons are either bound closely to the nucleus by electrostatic attraction or participate in metal-metal bonds with neighboring atoms. Substantial energy is required to dislodge these core and valence band electrons. Accordingly, the electrons do not flow when an electrical field is applied, i.e., they do not participate in the electric current.
Conduction band electrons in copper and other metals move much more freely under an applied electrical field than do electrons in an insulated material such as plastic. But, the electrons in the current randomly collide with each other and with the vibrating metal crystal lattice. These collisions dissipate energy, introduce a small but measurable resistance to the electrical current, and can induce unwanted noise (random information) into sensitive electrical devices that are based on traditional conducting materials.
In contrast, superconducting materials allow electrons to flow with no measurable resistance. The physics of such superconducting materials is usually explained by a quantum mechanical model in which vibrations of the crystal lattice form electron pairs, in which all conduction band electrons are thus paired up. When an electric field is applied along the conducting plane of a superconducting crystal, the electron flow is coordinated both with other electrons in the current and with the vibrating crystal lattice. This causes a coordination of flowing electrons, not only with the electrons, themselves, but with the crystal lattice thereby eliminating the random collisions that lead to resistance. The result is resistant free current, or in other words superconductivity.
The phenomenon of superconductivity has been known for decades in certain materials when they are cooled close to absolute zero. However, at this temperature such products are impractical for commercial applications.
However, beginning approximately 10 years ago, superconducting materials were discovered that operated at finite temperatures, e.g., 20-100 K. These materials became known as high temperature superconductors.
High critical temperature (high T.sub.c), superconductors are practical for commercial applications. For example, they can be cooled with liquid helium or inexpensive liquid nitrogen. Therefore they have been the subject of intense interest. In this regard, commercial products incorporating superconducting materials are well along in the development stage and even have begun to appear in the marketplace. An example of such a product is a commercial superconducting magnet offered by Cryomagnetics Inc.
A number of different materials (systems) have been found to have high temperature superconducting properties. These systems, for example, include: YBaCuO; TlBaCuO; TlSrCaCuO; TlBaCaCuO; and BiSrCaCuO.
Superconducting compounds with various compositions have been described in the literature for the fabrication of wires and tapes. Yttrium barium copper oxide in the composition Yba.sub.2 Cu.sub.3 O.sub.7, has the best flux pinning property at high temperature (77 K) of all the copper oxide materials known so far. However, a high temperature (up to 950.degree. C.), and the relatively long time required for the formation of the compound, as well as its ceramic nature, processing and the need for grain alignment (texturing) makes it difficult to draw in the form of long wires.
Thus, Tl-1223 has presently attracted attention to determine if it is a useful compound for fabrication into wires and tapes. The compositions that have been extensively studied are Tl.sub.0.5 Pb.sub.0.5 Sr.sub.1.6 Ba.sub.0.4 Ca.sub.2 Cu.sub.3 O.sub.9, Tl.sub.0.8 Bi.sub.0.2 Pb.sub.0.2 Sr.sub.1.6 Ba.sub.0.4 Ca.sub.2 Cu.sub.3 O.sub.9 and TlBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9. The former two phases require higher processing temperature. Also, single phase materials are never obtained and instead, Sr--Cu--O and Ca--Pb--O oxide containing phases are always noticed as an impurity with more than 15% abundance in the mixture. The latter phase has possible intergrowths of a Tl-2223 phase depending on the carbon content in the starting composition.
Thus, there is still a need for high quality high temperature superconducting materials that allow for an ease of formation at relatively low reaction temperatures making them more viable for practical applications.